The present invention relates generally to an improved spring and expander cap assembly and more particularly to such assemblies which are adapted for use in wheel cylinders of vehicle brake systems and a method by which such expander caps may be securely fastened to the end coils of a helical coil spring.
Drum type hydraulic brake systems such as commonly provided on the rear wheel of present day motor vehicles generally include a hydraulically actuated wheel cylinder comprising a housing having one or more pistons movably disposed therein. In some such wheel cylinders, a generally cup-shaped seal is provided which is urged against the inwardly facing end surface of the piston by a suitable coil spring. Originally, expander caps were not required on these springs because the brake system was filled with brake fluid and bled to remove air therefrom in a relatively conventional manner and the spring alone was sufficient to maintain the seal in position within the wheel cylinder. However, it has now become a common practice to initially fill the brake hydraulic system by first drawing a vacuum thereon and then introducing the brake fluid. Because the presence of a vacuum within the wheel cylinder will tend to collapse the skirt portion of these seals, it has become necessary to provide means such as expander caps on the ends of the springs to maintain the seals in an expanded condition and thereby prevent leakage of air into the brake system during filling thereof.
In order to facilitate assembly of the wheel cylinders as well as to insure the springs will be properly positioned relative to the associated expander caps, it is desirable to securely attach the expander caps to the springs so as to form a subassembly therefrom. However, in so forming the subassembly, it is very important that the expander caps be securely attached to the spring ends so as to prevent loss of one or both of the end caps during shipping or handling thereof.
Heretofore, such end caps have been attached to the springs by providing a plurality of nubbins on the expander cap. However, this method has not proved totally satisfactory. In the nubbin form of attachment, a cylindrically shaped flange portion of the expander cap is provided with a number of relatively small generally radially outwardly extending protrusions or nubbins spaced around the periphery thereof and the spring is pressed onto the flange portion so as to cause the end coil to snap over the nubbins. In this attachment arrangement, the tolerances between the spring and the expander cap are critical to the security of the attachment and hence often expander caps may become separated from the spring. While it may be possible to improve the security of this attachment arrangement by requiring closer tolerances on the manufacture of both the spring and expander caps, this would result in significantly higher production costs and therefore is not a completely acceptable alternative.
Dulude, U.S. Pat. No. 3,782,708, discloses another means of attaching a spring to a stamped plate by staking which method does provide a secure connection therebetween.
However, this method requires a tool to move coaxially through the spring to form a generally cylindrical flange over the end coil of the spring and thus could not be utilized to attach expander caps to both ends of the spring. In double action wheels cylinders, two pistons and two seals are provided which are movable in opposite axial directions and thus expander caps must be provided on both ends of the spring to retain each of the seals in position.
The method of the present invention, however, overcomes these disadvantages by providing a means for securely attaching the expander caps to both of the opposite end coils of a spring which method does not require excessively tight tolerances for the manufacture of either of the parts but yet provides a very secure attachment therebetween. In accordance with the method of the present invention, the expander cap is provided with a plurality of fingers or tabs which are designed to extend into the interior of the spring. These tabs are initially provided with a shape so as to allow the spring to be easily fitted thereover after which a suitable tool operates to deflect the fingers over the outermost or end coil of the spring. Because the tool enters the spring from the end to which the expander cap is being attached rather than moving axially through the spring as in the aforementioned patent, the presence of any restrictions on the opposite end of the spring (such as for example the other expander cap) will not interfere with the practicing of the present method. Further, it is possible to simultaneously attach expander caps to both ends of the spring thereby allowing rapid relatively low cost assembly thereof.
Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims taken in conjunction with the accompanying drawings.